The present invention relates to ceramic media, such as used in mass transfer applications and particularly to applications involving exposure to operating conditions containing halogens or halogen halides. Mass transfer, in the context of this Application, can mean separation of a component from a mixture of liquids or gases or the extraction of heat from a fluid flow. The ceramic media can be in the form of packing elements, such as those shapes commonly used in mass transfer applications, or other random or structured packing element shapes. The ceramic media could alternatively be in the form of bed support media. Without prejudice to the generality of their application in such fields, the ceramic media of the invention are particularly useful in the context of regenerative thermal oxidizers (xe2x80x9cRTO""sxe2x80x9d) in which a gas flow containing halogens, (typically chlorine, but with some bromine and/or fluorine components possible), and the correlative hydrogen halides.
RTO units are becoming more important as the drive to clean up effluent gases and to conserve energy becomes more urgent. In an RTO unit an effluent gas containing combustible or pyrolyzable materials is cycled through a first chamber containing packing elements that has previously been heated and thereafter enters a combustion chamber where the combustible or pyrolyzable materials are burned. The effluent gases then pass through a second chamber containing packing elements. These absorb at least some of the heat from the gases before the effluent is discharged to the atmosphere of for further processing. When the elements have reached an elevated temperature such that heat transfer no longer occurs efficiently, the flow direction is reversed and the second chamber becomes the first chamber and vice versa.
The packing elements in the chambers of the RTO can be in the form of monoliths with a plurality of through passages that are stacked within the chamber to provide a plurality of rectilinear parallel passages through which the gas can flow on its way through the chamber. Alternatively and often preferably the elements are relatively small individually and are dumped in random fashion within the chamber so as to provide a large number of non-rectilinear routes through the chamber for the gas. The individual elements can have a wide range of shapes such as hollow cylinders, with and without internal septa or other internal structures, cylinders with triangular or xe2x80x9cbow-tiexe2x80x9d cross-sections, and porous pellets.
Gas flows that are particularly suitable for treatment using RTO""s may be generated for example when gas flows containing combustible materials that include halohydrocarbons are burned in an RTO unit as part of an effluent purification process. In such applications it is necessary that the elements used in the RTO are capable of absorbing heat rapidly and are stable under thermal cycling conditions as would be expected, but also that they are resistant to attack by the halogen-containing components of the effluent. This is important since replacement of the packing elements usually requires a shutdown of the RTO while the elements cool, are then extracted and replaced. Obviously the fewer times this has to occur, the better and more economically the unit operates. Furthermore, in the cases in which the ceramic elements in the RTO""s are attacked by the halogen-containing components of the effluent, but not so severely as to degrade the media to the point of necessitating a change-out, there are still problems that result directly from the ceramic-halogen reactions. The reaction of the halogens, such as Cl2 and other chlorine-containing gases with the elements inherently present in ceramic media, especially Na, K, and possibly Li, Ti and Fe, results in the formation of a precipitate, particularly NaCl and KCl, downstream from the thermal oxidizer. In the cases with downstream waste-heat boilers, this deposition of the precipitate causes a buildup and eventually fouling of the boiler, which causes a shut-down of the whole process for a clean-out.
Typical packing elements for RTO applications are made from clay/feldspathic material mixtures because these have good stability to thermal cycling while having a good capacity to absorb heat. They are, however, seriously attacked by atmospheres containing halogens or halogen acids. The present invention provides packing elements that are relatively stable to attack under such conditions and which therefore provide a significant advantage for treatment of hot halogen or hydrogen halide-containg effluents from catalytic or other processes for making or treating halogen-containing organic compounds.
In accordance with one aspect of the present invention, ceramic media is provided. The ceramic media includes 40-65% by weight SiO2, 10-45% by weight Al2O3, less than 0.25% by weight of alkali metal, expressed as the oxide, and less than 31% by weight alkaline earth metals, expressed as the oxide. The ceramic media has a ratio of cordierite: crystalline silica phase of less than 1.2, as determined by X-ray diffraction analysis.
In accordance with another aspect of the present invention, ceramic media includes 40-65% by weight SiO2, 10-45% by weight Al2O3, less than 0.25% by weight of alkali metal, expressed as the oxide, less than 31% by weight alkaline earth metals, expressed as the oxide. Less than about 40% by weight of the media is in the form of cordierite.
In accordance with another aspect of the present invention, a process for the production of ceramic media is provided. The process includes forming a mixture comprising 10 to 98%, by weight of a clay having an alumina content of at least 36% by weight; from 2 to 90% by weight of a talc containing at least 95% by weight of magnesium silicate as determined by X-ray diffraction analysis; and from 0 to 10% by weight of a dolomitic limestone containing 60 to 90% by weight of calcium carbonate and 10% to 40% by weight of magnesium carbonate and less than 10% of non-carbonate impurities. The mixture contains less than 0.25% by weight of alkali metals, measured as the corresponding alkali metal oxides in the product remaining after firing. Water is added in an amount sufficient to make a shapeable product and the shapable product is formed into to a desired shape. The shaped product at a temperature of from 1100xc2x0 C. to 1400xc2x0 C. for sufficient time to form ceramic media having a ratio of cordierite:silica phase of less than 1.2.
xe2x80x9cCalcinationxe2x80x9d as used herein, refers to a heating process to which a material has been subjected. Ore materials that have been fully subjected to calcination or a calcining process exhibit very low loss on ignition (LOI) and moisture contents, e.g., about 1-2 percent by weight or less. Uncalcined ore materials such as bauxites and clays can contain from about 10 to about 40 percent by weight volatiles. xe2x80x9cPartially calcinedxe2x80x9d materials typically exhibit total volatiles (LOI plus moisture content) of 5 to 8 percent by weight. Volatiles can include moisture, organics and chemically held water (e.g., water of hydration).